insulating material info - please...

I have propane heat. It breaks my bank every winter. The ductwork under my house in the crawlspace is the old sheet metal type. I want to insulate it to possibly help the system run more efficiently. What concerns me is that sometimes in the summer condensation can build up on the duct work when it is really hot and the ac is blowing. I want to avoid mold buildup on the insulation material I use. What do I need to know about the insulation material I use?
Next question is pipe related. I have copper pipes under the house. If I insulate them, I only have to insulate the hot water lines right? (frozen pipes are pretty much unheard of down here)
Last, the water heater is under the house. It is the lowboy type b/c the crawlspace is only 3 feet there. Is it really worth wrapping the heater with insulation?
I am always grateful for the advice given here.
Jeremy

Spray foam (open or closed cell) is what you'd need on the ducts to avoid condensation with a ventilated crawlspace, but air-sealing and insulating the crawlspace would be a better solution, at which point sealing the seams & joints with duct-mastic and using R6 fiberglass duct insulation would be fine.

Is the water heater electric, or propane? Most electric tanks have pretty good insulation, most gas/propane not so much, but care must be taken not to create fire or exhaust problems when retrofitting them.

All near-tank plumbing (within 6-10', even the cold) needs to be insulated with at least 5/8" closed cell foam pipe insulation (3/4" is better). The stuff they sell at box stores is only 3/8"- better than nothing, but at the price of propane or electricity it's cost-effective to double that. In a vented crawlspace it's probably worth putting at least the cheap 3/8" goods on the rest of the cold plumbing- it won't keep it from freezing but it will have a very modest return on energy use. If you go ahead and seal & insulate the crawlspace you can skip insulating the cold feed.

In NC vented crawlspaces INCREASE rather than decrease the humidity level of the structural wood in the crawlspace, since the summertime dew points are much higher than the ground temperatures. In a air conditioned house and have insulated the floors this can become a serious mold hazard, since the cold subfloor &/or joist edges can all be well below the dew point of the outdoor air to which it is exposed.

Okay, a lot of that went over my head.
I'm not sure what air-sealing and insulating the crawlspace entails, but it sounds expensive. We had the floors insulated a couple years ago. There is black plastic down above the dirt in the crawlspace as a "vapor barrier". There is definitely a lot of moisture in the clay earth in the crawlspace but aside from spending a few thousand dollars, we can't do anything about it.
The water heater is electric.

In NC vented crawlspaces INCREASE rather than decrease the humidity level of the structural wood in the crawlspace, since the summertime dew points are much higher than the ground temperatures. In a air conditioned house and have insulated the floors this can become a serious mold hazard, since the cold subfloor &/or joist edges can all be well below the dew point of the outdoor air to which it is exposed.

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How do I increase the humidity level of the structural wood in the crawlspace? I think it is already pretty humid. The crawlspace vents stay open.

If you crawlspace is not sealed, the vents themselves add to the humidity level (which is bad). To lower it, you'd have to seal up the crawlspace...sort of like making it living space. The vapor barrier on the ground helps, but there are other things you can do. If you block moisture from coming up from the ground, and don't have humid air coming in, you can control the humidity level under there. With it open, the moving air with the high normal summer humidity levels, raises it.

To keep from growing mold, you'd have to also insulate and seal to prevent moisture from migrating in. The details can be critical, otherwise, you then make matters worse.

Okay, a lot of that went over my head.
I'm not sure what air-sealing and insulating the crawlspace entails, but it sounds expensive. We had the floors insulated a couple years ago. There is black plastic down above the dirt in the crawlspace as a "vapor barrier". There is definitely a lot of moisture in the clay earth in the crawlspace but aside from spending a few thousand dollars, we can't do anything about it.
The water heater is electric.

How do I increase the humidity level of the structural wood in the crawlspace? I think it is already pretty humid. The crawlspace vents stay open.

Click to expand...

The open vents are a problem, since they let in humid outdoor air to condense on the joist edges and ducts for much of the summer, and part of the spring & fall. The solution is to make it air-tight to the outdoors, with a bit of insulation on the walls & band joists. If the floor insulation you installed isn't truly air-tight you may have created mold conditions at the subfloor and in the insulation too. It's not cheap, but it's almost always worth it in the long run.

Just closing up t the foundation vents tight (with good weatherstripping if you make them operable, if required by code) sealing the foundation sill to the foundation & band joist with 1-2" of closed cell foam, and using duct-mastic to seal the edges of your ground vapor barrier to the foundation (if it isn't already), will likely reduce condensation on the ducts, even without insulating the foundation walls. By sealing it all up the absolute humidity (==dew point) of the crawlspace air will track that of the conditioned space despite being colder. Summertime humidity issues would be much abated, but then it becomes a matter of what happens in winter. If the crawlspace stagnates for weeks on end below ~40F (not TOO likely since that's about the average binned hourly temp in January for Durham) there could be wintertime moisture accumulation issues but as long as it's sealed from bulk water penetration you're probably good. If you put even R8 (2" of rigid EPS/bead-board insulation) on the foundation walls and seal the seams tight it'll likely stay above 50F (maybe above 60F) all winter long. R12 using 2" of closed cell spray foam or 3" of open cell would be more expensive, but would also provide a more assured air-seal.

The fact that your HW heater is electric means you don't have to provide for combustion air, which makes things easier.

It's often possible to find factory-seconds or reclaimed rigid insulation at less than 1/3 the cost of retail. Pink/blue/green/gray XPS is pretty good stuff- more rugged than EPS and up to 2" would be OK to use in this application. Be careful about using anything with a foil, vinyl, or poly facer on it, since that would limit the foundation's ability to dry toward the interior, and if it wicks too much ground moisture up to the foundation sill it could raise the humidity of that wood. (If there's a good sill-gasket blocking that moisture path that's less of an issue.) A quick craigslist search in my area usually shows multiple sources of reclaimed EPS from commercial re-roofing, but I didn't see any in your area.